Oh yes, I went through this too during my second trimester, I remember panicking when I noticed my brows looking so sparse in photos! From what I have heard and experienced, it is often linked to hormonal changes, but it can also point to a nutrient deficiency or thyroid levels being a little off, so it is worth mentioning to your gynae or GP at your next visit just to be safe. Some mums I know noticed the thinning more on the outer part of the brow, which apparently is a common sign when thyroid levels are not quite right during pregnancy. My neighbour actually brought this up with her doctor around her third trimester and they did a quick thyroid check, which helped them catch a mild imbalance early, so do not just brush it off as "normal pregnancy stuff." Making sure your prenatal vitamins and diet are giving you enough nutrients can also help, as deficiencies do play a role. Hang in there, you are not alone in this, and getting it checked early will give you real peace of mind!